Such drive systems are employed, for example, in electrically operated vehicles or hybrid vehicles, such as automobiles, buses, trains, etc. which drive over land and are commonly referred to as land crafts or land craft vehicles. Such vehicles typically have one or more driving wheels which are driven by the drive system to move the vehicle or craft in the desired movement direction. For example, the electrical energy which is necessary for the drive is supplied by a combustion engine running an electrical generator, by a fuel cell, by an accumulator battery, or by a combination of such devices acting as an electrical source. The electrical source is connected to the electrical machine system of the vehicle for driving purposes. The electrical energy of the electrical source is generally supplied to a supply circuit including power electronics, for example in the form of an electric intermediate circuit having converter circuits on both sides, which is connected to the electrical source and to the electrical machine. The electrical machine of the vehicle is supplied by the supply circuit with electrical energy in order to drive one or more driving shafts which drive the driven wheels of the vehicle. In most applications, the drive system can also carry out electrical braking with recovery of energy so that it feeds electrical energy back into the supply circuit. This energy can be consumed by other devices which are connected to the supply circuit. For example, it can be stored in an accumulator battery or in a flywheel accumulator.
The driving of electrically operated vehicles typically requires mechanical power which must be generated from electrical power with a high spread having a hyperbolic characteristic. Such hyperbolic characteristic is typically characterized by a high maximum rotation speed of the electric drive machine and a high torque at low rotation speeds, each defining a characteristic point of the hyperbolic characteristic which are connected through a hyperbola with equal power. In mechanically driven vehicles, for example driven directly through a combustion engine, this function is taken over by the manual or automatic transmission or gear box. In driving an electrically driven vehicle, this function must be assumed by the electrical drive system, particularly by the combination of the power electronics coupled to the energy source and the electric machines. The converter circuit feeding the intermediate electrical circuit is usually not a problem and may be optimized regarding various purposes. For combustion engines, there increasingly exist permanent-magnet machine generators directly integrated to the engine which have a high degree of efficiency and are coupled to active or passive converter circuits. However, the motor unit reaches its limits with various possible fields of applications and realizations of such drive systems, particularly with respect to machine size, degree of efficiency, and electrical requirements of the power electronics regarding maximum current and voltage values.
In the recent state of the art, there exist various types of machines which may be applied in an electrical drive system as set out above. For example, a permanent-magnet (PM) machine may be applied which is supplied by a converter circuit. The PM provides very high torques per weight, is capable for a direct drive, but on the other hand requires high demands for the power electronics. Since the induced voltage (so-called “EMF”_(Electromagnetic Force) or “Back-EMF”) is proportional to the rotating speed, very high voltages occur with maximum speed, on the other hand high driving torques at low speed require high currents. The power electronics must be capable to accept the high voltages at high speeds and to provide high currents for high torques, which makes the power electronics very power demanding and expensive. The so-called electrical corner power (maximum voltage multiplied by maximum current) is identical with the mechanical corner power (maximum rotation speed multiplied by maximum torque).
Such high corner powers make the power electronics very power demanding and expensive since they have to provide high voltages as well as high currents to the machine. On the other hand, since maximum voltage and maximum current are never demanded by the machine at the same time, the power electronics provides an over-installation metered by the corner power as opposed to the nominal power (hyperbolic power) of the drive unit. Low requirements for the power electronics correspond to corner power which equals nominal power, high requirements for the power electronics correspond to corner power which equals maximum torque multiplied by maximum speed. In typical applications, these extremes may differ from each other by factors of between 4 and 10.
Regarding the permanent-magnet machine, the high requirements of the power electronics are often reduced by increasing the winding number of the machine, however leading to increasing induced voltages of the machine. For example, within 700 V DC systems, the machine maybe typically configured up to 1500 V induced voltage. From approximately half rotation speed on, these machines must be operated permanently with the correct phase shift to keep the terminal voltage below critical limits in order to be able to control operation of the machine. This may be realized through so-called field weakening operation with active power electronics and by means of additional safety circuits which make sure that the machine may still be operated when the power electronics fail in order to avoid high voltage load to the system. This additionally makes the drive system expensive and demanding.
The optimization of the PM machine regarding compactness and, at the same time, degree of efficiency at partial loads is only possible within limits, since for achieving high torques per weight of the machine correspondingly strong magnets are required which generate a constant high degree of losses also at partial load.
Other known drive systems employ an asynchronous machine coupled to a frequency converting circuit. This combination maps the hyperbolic characteristic well which results in rather low demands for the power electronics. However, asynchronous motors provide rather low torque per weight, thus are not that capable of serving as a direct drive. For this reason, asynchronous motors are in many cases combined with transmission gears which results in additional rotational losses.
Drive systems also employ a combination of reluctance machine and converter circuit, which may be applied as a direct drive. However, with high rotational speeds the power decreases more than the desired hyperbolic characteristic of equal power. Further, the compactness is rather low (PM machines can be constructed with approximately half of the dimensions), the degree of efficiency is in mid-range and there are low demands for the power electronics.
As a further type of electrical machine, a so-called IPM machine (“internal or interior permanent-magnet machine”) supplied by a converter circuit is employed in drive systems as mentioned above. The IPM machine is a derivative of the PM machine having magnets which are embedded in the rotor, thus providing a magnetically slotted or toothed rotor for generating additional tension forces when rotating the motor. On the other hand, the IPM machine has a relatively complex rotor construction so that it may be realized only with a reduced number of poles as compared to the PM machine, which increases the weight of the machine. The demand for the power electronics may be decreased as compared to the PM machine for about 20-30%, which is nevertheless still high.